1. Field of the Invention
The present invention relates to an inspection apparatus for optically inspecting working surfaces, and more particularly to an optical inspection apparatus having a dynamic spatial attenuator.
2. Description of Related Art
The silicon manufacturing process requires a pure environment free from contaminants that often produce undesired and unexpected effects on device performance. These effects can range from damaged photomasks to unwanted electrical properties in the case of semiconductor chips, and from physical obstructions to weakened mechanical properties in the case of nanofabricated machinery. Maintaining a pure manufacturing environment requires the detection of such contaminants on the device surface before their elimination can begin.
Most contaminants on the device surface are too small to be seen by the naked eye. Hence, one approach to detect them is to use an inspection apparatus that illuminates the substrate""s surface with a laser beam. If a contaminant exists on the substrate""s surface, it will scatter light in all directions. The intensity of the scattered light depends on a number of variables, such as the contaminant""s size, the contaminant""s and the substrate""s refractive index, the illumination angle, the collection angle, and the illumination source""s wavelength and intensity. Photo-detectors strategically placed around the hemisphere above the substrate will detect the scattered light and convert it into an electrical signal for further analysis and classification. Inspection of an unpatterned substrate with such an apparatus is simple because the electrical signal is constant if the unpatterned substrate is clean and free of irregularity. Conversely, any sharp deviation in the electrical signal""s amplitude may be attributed to an undesired surface condition such as the presence of a contaminant, surface defect, or other irregularity on the surface that causes an interruption in the pattern of the scattered light. Further attention would be required to determine the nature of the surface condition and to identify whether it is defect related.
Inspecting a patterned substrate is more complex than inspecting an unpatterned substrate because scattered light produced by the pattern""s topography is mixed in with light scattered by a possible contaminant, defect or other undesired surface conditions. Most signals produced by the patterns, however, are periodic and can be filtered out by computer hardware or software, leaving behind the non-periodic signals for further analysis. If the periodic features are much smaller than the spot size of the illumination laser beam (i.e., an array of cells in a memory area), these features will behave like a reflective diffraction grating and produce a Fourier diffraction pattern. One method to remove this Fourier diffraction pattern, thus enhancing the contaminant or defect""s signal vis-à-vis the Fourier diffraction pattern""s signal, is to employ a Fourier transform lens to focus the diffraction pattern onto a spatial filter located on the Fourier transform plane.
Several methods of filtering this Fourier diffraction pattern exist in the art. One method uses a spatial filter comprised of a planar array of individually addressable light valves using transparent liquid crystal technology (see, U.S. Pat. No. 5,276,498 to Galbraith et al.). Technology based on transparent liquid crystal is inefficient when compared to reflective mirrors due to the material""s opaqueness. A second method uses a photographic plate to capture the Fourier diffraction pattern and translate the photographic negative into a spatial filter. This approach requires physically changing the filters for different substrates and is impractical for current manufacturing processes. A third method uses a spatial filter consisting of mechanically linked bars positioned to block the diffracted light (see, U.S. Pat. No. 5,742,42 to Drake). This approach is limited by its resolution. A fourth method uses a micro-mirror array as a spatial separator to direct different components of the diffraction pattern to different detectors (see, U.S. Pat. No. 5,506,676 to Hendler). This approach requires multiple photo-detector subassemblies of varying sensitivity and is costly to implement.
Further, care must be exercised to ensure that the intensity of the scattered light from the working surface does not exceed the designed detection limits of the detector (i.e., saturate the detector). Otherwise, when the detector is saturated, it would no longer be able to distinguish between two different surface conditions. The sensitivity of the detector may be reduced to accommodate detection of a wider range of intensities, but that would mean that the apparatus would be less sensitive at lower intensity levels. In the alternate, the intensity of the scattered light may be reduced.
For example, a change in the collection angle relative to the illumination angle will affect the intensity of the scattered light, and hence the sensitivity of the inspection apparatus. Several methods of varying the collection angle relative to the illumination angle exist in the art. One method uses a mechanical curtain or a transparent liquid crystal light valve to progressively cut out part of the collection window, which results in changing the solid collection angle. This approach is slow and cannot be changed during mid-scan. Another approach physically raises or lowers the collection assembly or its subassembly. This approach requires precise optical realignments and is costly.
What is needed is an improved optical inspection apparatus that can dynamically and spatially attenuate the scattered light from the working surface, especially scattered light from known surface""s topography, without compromising detection sensitivity.
The present invention provides a simplified, low cost, efficient, reliable and stable optical attenuator that attenuates the intensity of the scattered light from a working surface with respect to a detector. The present invention is particularly suited for deployment in an optical surface inspection apparatus for identifying unknown surface conditions of a working surface based on detection of scattered light from the surface.
In one aspect of the present invention, a dynamic spatial attenuator that is reflective in nature is deployed in the collection optics of an optical surface inspection apparatus. The attenuator is selectively controlled to vary the amount of scattered light, so as to vary the intensity of the scattered light reaching the detector. By adjusting and optimizing the intensity of the scattered light with respect to the detector, the detector can operate with an increased resolution and at the highest sensitivity possible.
In one aspect of the present invention, the dynamic spatial attenuator comprises a two-dimensional array of reflective surfaces, which may be individually controlled to divert a desired amount of scattered light away from the detector, resulting in a decrease in the intensity of the scattered light reaching the detector. In one embodiment, the reflective array comprises a two-dimensional micro-mechanical reflective array, made up of tiny moveable reflective elements. Each individually addressable element can be tilted to attenuate the scattered light that reaches to different destinations. The reflective array may be controlled to tilt the individual reflective elements to a position such that scattered light is selectively diverted to the detector by some of the reflective elements in the array (hereinafter referred to as an xe2x80x9conxe2x80x9d state or fully xe2x80x9conxe2x80x9d state since there is only one position in which the reflective elements divert light to the detector), and other reflective elements are diverted away from the detector (i.e., in an xe2x80x9coffxe2x80x9d state, with a fully xe2x80x9coffxe2x80x9d state in which light is diverted away from the detector and to a light dump). A desired attenuation pattern can be dynamically configured on-the-fly during the optical inspection process.
In one embodiment, by selectively controlling a predetermine number of reflective elements in the array to divert light to the photo-detector, the desired level of attenuation is achieved. In another embodiment, the desired level of attenuation is achieved by controlling the relative xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d durations in which some or all of the reflective elements are diverting light to the detector and light dump. For example, the reflective elements may be controlled to flicker at a regular cycle at a frequency that results in the desired average duration in which the reflective elements are in the fully xe2x80x9conxe2x80x9d state at which light is being diverted to the detector. Because the elements are in the xe2x80x9coffxe2x80x9d state when light is not diverted to the detector (the elements do not need to be at the fully xe2x80x9coffxe2x80x9d state to divert light away from the detector), the duration of the xe2x80x9coffxe2x80x9d and xe2x80x9conxe2x80x9d duration are therefore not equal in a regular cycle. Alternatively, the reflective elements may be controlled to alternate between xe2x80x9coffxe2x80x9d and xe2x80x9conxe2x80x9d in an irregular cycle in a manner where the reflective element is diverting light to the detector (fully xe2x80x9conxe2x80x9d) at a first portion of a flicker cycle and diverting light away from the detector (xe2x80x9coffxe2x80x9d) in a second portion of the flicker cycle, whereby the duration of the first and second portions of the cycle maybe equal for a 50% attenuation, or unequal for a less than or greater than 50% attenuation. The distribution of reflective elements that are in the xe2x80x9coffxe2x80x9d and xe2x80x9conxe2x80x9d states (attenuation pattern) may be in a regular pattern or irregular pattern over the array. A combination of the foregoing schemes may be implemented to obtain the desired attenuation effect to prevent the detector from being saturated without compromising detection sensitivity. The reflective array may be reconfigured dynamically from one attenuation pattern to another during an optical inspection process to optimize over different areas of the working surface.
In another aspect, the reflective array may be selectively controlled to create an attenuation pattern that filters Fourier diffraction patterns created by regular surface features on the working surface.
In another embodiment, the reflective array may be controlled to vary the collection aperture (and thus the collection angle) without physically reconfiguring the detection optics to vary the collection angle relative to the illumination. By selectively turning an area of the reflective array xe2x80x9coffxe2x80x9d (e.g., by turning off adjacent bands of reflective elements), part of the reflective array is disabled from diverting light to the detector, thus altering the solid collection angle. The collection angle may be altered with respect to the working surface and/or with respect to the illumination angle by selectively turning xe2x80x9coffxe2x80x9d the rows and/or columns of the reflective array, as referenced to the working surface.
In a further aspect of the present invention, the entire micro-mechanical reflective array can be divided into sub-arrays of elements. For example, a sub-array of 100xc3x97100 elements will create a total of 10,000 reflective elements that can be turn on or off as a single unit to decrease the complexity of the controlling algorithm.